I'm sure similar questions will pop up over time after Toy Story Land and STGE open up. I have to assume the answers will be the same as for Pandora.
Well, obviously, one shouldn't even bother going to Disney World at all unless they have seen:
Cinderella
Peter Pan
Snow White and the Seven Dwarfs
Fantasia
Beauty and the Beast
The Little Mermaid
Sword in the Stone
Toy Story 1 - 3
Haunted Mansion
Pinocchio
Alice In Wonderland
20,000 Leagues Under the Sea (What? When? Boooo! Okay, scratch that one.)
Princess and the Frog
Sleeping Beauty
The Many Adventures of Winnie the Pooh
The African Queen (not Disney but still - I mean, everyone should see that one anyway)
Lilo & Stitch (or wait, you can skip that one now)
All 5 Pirates of the Caribbean Movies
A Swiss Family Robinson
Song of the South
Aladdin
Dumbo
Country Bears
Monsters Inc./Monsters University
Frozen
Tangled
Finding Nemo & Finding Dory
The Three Caballeros & Saludos Amigos
Mission to Mars
At least 7 episodes of The Twilight Zone
Pocahontas
All 4 Indiana Jones Movies (soon to be 5)
The complete discography of Aerosmith
Star Wars, Star Wars, and more Star Wars
Avatar
Avatar: Extended Edition
Avatar: Extra Extended Edition (and know the difference - there's a quiz!)
Dinosaur
Lion King (and the sequels maybe?)
Go ahead and get a head start on Ratatouille and Guardians of the Galaxy Volumes 1 and 2 since that's coming our way. Oh, and Tron and Tron Legacy.
Okay, some of those are less important than others, but if you eat in a themed food venue, there's a back story!
Seriously, that's why I love Disney. It permeates our lives so much, and yet they do a great job in the parks of making everything accessible whether you have seen the movies or not. Rest assured it's an enjoyable from the most hardcore Disney Nerd to someone who doesn't know who Figaro is. It's fun for all!